text:
How can I figure out critical altitude?
In Vmc situation (minimum speed where pilot can maintain altitude), How can pilot figure out the Vmc? And also, why critical altitude is important to know?

That should all be jn the operations manual.

If you have 2 questions (the Vmc and the critical altitude), you ask them separately

Vmc is not the speed in which you can maintain altitude, it’s the minimum speed in which you can still maintain directional control with an engine inoperative in a twin.  Vmc is a design specification and not a maneuvering speed which is established by the manufacturer according to the design requirement of the aircraft.  The actual minimum control speed for a given situation, Vmca, varies based upon a number of factors, including density altitude, loadout, aircraft configuration, etc.  Vmca can be determined by doing a Vmc demonstration, which is a required maneuver to be demonstrated for an airman certificate operating multi engine airplanes.
Critical altitude is the altitude at which above this a turbocharger or supercharger can no longer maintain the maximum design operating manifold pressure for a reciprocating engine.  It is an important factor if you need to know the maximum power output of your engine at any given altitude for climb requirements, ability to attain and maintain an altitude to clear terrain and obstacles, etc., no different than knowing the available power for an normally aspirated engine.

Critical altitude is the DENSITY altitude above which..., not the pressure altitude above which...


text:
How to calculate service ceiling and absolute ceiling in PA-44 Seminole?
I am hard to understand to calculate service ceiling and absolute ceiling in PA-44 Seminole. Is there anyone know how to calculate it in the condition(T/O weight: 3599.24 L/D weight: 3354.44 PA:2329 Temp:20)

You can use the given chart to define (single engine) ceilings. There should be separate graph for both engines running case.
Note: it is important to understand that the definition absolute ceiling is absolute: zero climb rate while service ceiling is defined by local regulations.
In Europe, in commercial operations for class B aircraft (which Seminole is) service ceiling is the altitude where both engines running aircraft is able to climb at 300 ft/min at present gross weight and local OAT. A single engine service ceiling is defined as "positive gradient" so it is the first usable altitude below single engine absolute ceiling.
To define required altitude enter the graph from the right from the desired vertical speed, go upwards until you meet your current (or estimated) gross weight. From there draw a horizontal line all the way across the left side of the graph. The point where to local OAT (at an altitude) and your horizontal lines cross is the ceiling according to the selected climb requirement.
When interpolating between the altitude lines make sure to go perpendicular between altitude lines - not up and down along the grid.

Service ceiling I believe is the altitude at which an aircraft will no longer climb at a rate of 100 ft/minute.  It should be published in the POH for the plane.
For example, in a 1977 Cessna 150

SERVICE CEILING .....  14, 000 FT

That is a single engine, 2 seat propeller plane, with no turbocharging/turbonormalizing.
The Seminole POH should have a similar number.  Absolute ceiling can be higher but it will take a while to get higher. Use of O2 is required above certain altitutes, and after certain time periods above a lower altitude as well.  If one departed a hot California airport at 45C (110F, definitely possible out in the desert areas), say Rosamond Skypark (L00) at 2415ft, then 14000-2400 = 11,600 feet x 2C/1000 ft = 23.2C, and 45C - 23.2C = 21.8C!
Will feel nice & cool while airborne, but really hot when that door is opened.
We were flying the US southwest a few years ago, July 4th weekend. Flying southeast away from Albquerque, New Mexico, to Carlsbad, Arizona, we couldn't climb above 9,000 ft, my 180 HP normally aspirated engine just couldn't make any more power to get higher. So the rated Service Ceiling for my plane,

SERVICE CEILING ------------ 14,600 FT

made no difference. 107F on the ground.  Hot!

Thank you so much!

to acheive a density altitude of 14,600 at a pressure altitude of 9000 you would need an OAT of 50c(122f); the ground in Carlsbad is about 3200 which implies the ground air temp would be about 61.6c(143f) (9000-3200=5800, 5.8*2c=11.6c)
Allowing for a rather low(storm low) altimeter setting 29.00 this is a pressure altitude of 9862ft and would need an OAT of 39.5c for a density alt. of 14600, and an implied surface temp of 51c(124f). In Pheonix (yes slightly different, just data I have), the average low altimeter setting is 29.6 and the average high in summer 30.0. (add 0.6 to winter highs)

With the chart you uploaded you don't calculate the ceiling, but the climb rate when you have only one engine operating. The TO weight and LD weight don't matter, what you need is the GW (which should lie somewhere between the TO weight and the LD weight).
I have added red lines to the chart to show you how to use it:

Select your OAT (20°C in your case)
Move up until you meet your PA (2329 in your case)
Move right until you meet your weight (let's say 3599 at take off)
Move down and you get your climb rate (in this case 120ft/min)*

*At your LD weight it is 190ft/min
NOTE: at the bottom of the chart you see dashed lines with arrows. Those are used to understand the order in which you have to read the chart.

Thank you so much!

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

sort: votes


text:
How can I know DME distance?
When I fly 12000ft, how can I know a distance from DME station?
Is there any formula to know or measure distance from DEM?

Are you asking about how to calculate the difference between slant distance and ground distance? If so that’s just simple trigonometry.

Can you give a specific example of what you're asking? Are you asking how to calculate DME if you know both your altitude and horizontal distance from the navaid? Or are you asking how to calculate your horizontal distance from the navaid if you know your altitude and DME? Or something else?

If your radio has DME installed, then it is shown on the instruments

Just to be clear, when you are asked for your DME from a station or using a DME fix, what is being expected is that you use what you see as the distance in nautical miles on the DME readout of your instrumentation. You're not expected to use your altitude to come up with the distance along the ground using trigonometry.
It's the slant distance they're expecting you to report and use regardless of your altitude and the actual distance over the ground.

I lifted the image off a Google search for "images of dme instrumentation", and they got it from https://www.cfinotebook.net/graphics/avionics-and-instruments/distance-measuring-equipment/DME-Slant-Range-Distance.jpg

DME is a slant distance.  If you are asking for a lateral distance, that could be done using simple trigonometry taking the slant distance and field elevation of the DME station, and aircraft altitude to compute an azimuth from the station, thence a lateral offset can be computed.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
How does pendulous vane work?
The pendulous vane uses precession to correct a gyro by small jet air, but I   do not know how it works. As far as I study, when gyro is tiled, one of the doors open and the other one is closed. This principle seems to be hard for me to understand. Could you let me know the principle of pendulous vane, and how it corrects a gyro's error?

Nice diagram here: https://aviation.stackexchange.com/questions/16376/how-are-attitude-indicators-kept-accurate/16417#16417

There's an excellent explanation of them here if you can ignore the writer's delusional conspiracy ideas in the last paragraph


text:
Does anybody know that why C172S POH doesn't have normal & soft field landing distance chart?
Thank you for reading my writing.
I found out that C172S, C172P model POH doesn't have normal landing distance & soft field landing distance chart or table. It has only short field landing distance.
Does anybody know the reason?
Thanks.

Typically light GA aircraft do not list normal landing distances or soft field distances in AFMs.  Looking at the POH from a 172S

Look at Note 3, which refers to typical soft field conditions.  It indicates that a 45% increase in the short field figures is representative of soft field landing distances.
As a “normal” landing distances vary depending on how aggressive one applies stopping techniques on the ground this distance is going to vary depending on how the pilot flies the approach and landing.  Unless a specific procedure is described for this, as in the case for short field operations, there’s no way to draw up definitive performance tables for this.  In addition, the critical value here is if the pilot has enough runway available to fly the approach and stop the airplane in the minimum distance required.  If in doubt as to how much runway one will need during landing, the pilot should plan on using a short field procedure and use the published tables accordingly.
For general operations, I find an approximate 30% increase in the figures listed in these tables will provide a good margain for error and offer a conservative margain for landing figures.
Example: grass field, 4120 ft ASL, local altimeter setting of 29.83” Hg, 82° F (28°C), still air.
Pressure altitude is 4210 ft.
Using an E6B, the density altitude is 6,500 ft
Select 7000 ft from the table, 10°C column.
This gives a 730 ft landing roll from the table and 1585 ft distance to clear 50ft obstacle.
Increasing the ground roll figure by 45% for soft field gives 1059 ft ground roll and (1585-730) + 1059 = 1914 ft to clear 50 ft obstacle.
Increase by 30% for safety margin gives 1376 ft ground roll and 2488ft to clear 50ft obstacle.

But older airplane POHs have normal landing distance chart or table. I guess I need normal landing distance chart more than short field landing chart. Do you know why manufacturer stop publishing normal landing distance chart?

Which airplane specifically has a normal landing distances published?

@이준세 Because if you are concerned about the length of the field, you should be using the short-field technique. Also, don't land at a field you can't take-off from, the take-off distance is larger than the landing distance, so if you want a "normal landing distance" yard-stick, use the take-off distance chart.

Oh yes, the old manuals written in the days when many companies are not held legally liable for omissions like that.  No specification given for braking or stopping, etc.  As such the data is virtually useless.

Also, if you’re going to get a ground roll of 520ft at sea level on landing for a Cessna 172, that’s, by default, a short field procedure.  I have enough hours of PIC time in that airplane to know that.  I suspect back in those days it was conveyed to pilots by word of mouth from instructors that the data was to be interpreted as valid for a short field landing only.

Take a look at the numbers published from that old POH and look how close they match the numbers for a new model C172S short field landing performance in the table I posted above.  Again without a specified braking and stopping procedure, one person’s interpretation of a normal landing procedure varies quite a bit from another pilot’s.  It appears as though Cessna wrote the old tables intending pilots would use them for providing realistic figures for landings conducted using a short field technique only.  Also note the old tables just say “Landing” not “Normal Landing”.

Thank you Ron, Carlo, 757toga. Now I understood why POH has short field landing distance chart only. I am really really appreciate your precious answers. Thanks A LOT. Thanks.


text:
For a modern airliner, wings are tested to withstand 150% of the limit load. What kind of weather would produce turbulence at 100% limit load?
I often see people citing the wing flex tests, but with very little reference as to what kind of forces they're designing the plane to take.
I am specifically referring to modern day jetliners such as the Airbus A330.
The wings are tested to withstand 150 of the ''most extreme'' forces the plane will encounter. But what does this mean exactly? Not too long ago, a 777 experienced extreme turbulence, and the aircraft experienced vertical accelerations of +3 G. Would that have still been under limit load for the 777?

Not a duplicate. This is my first time asking this, thank you. This question is also about turbulence, not a course reversal.

At a high level it doesn't matter much whether the load is from maneuvering or from the environment. The regulations cited in the accepted answer to the other question are based on gusts and turbulence.

@WillyA, duplicates are when a question has already been asked by anyone, not a single person. In any case I would agree it's a different question.

The 1.5 relates to the margin between permanent bending (limit or yield load) and breaking (ultimate or breaking load).  Transport Airplanes are good to 2.5 limit load at gross weight.  Pulling more than that may or may not bend the wings permanently.  For ultimate load (pulling the wings off, it's 3.75Gs minimum using the normal 1.5 safety factor)
Those are minimums.  There is a lot of safety margin in the stress calculations so I would not be surprised at all to see an airliner take 3Gs without permanent damage even at max gross.  If the airplane is below its maximum zero fuel weight (a function of the pax and baggage load, which is where the wing bending is coming from) there is more margin, depending on how light you are.
Then there are the two speed limits to protect the structure to consider.  Maneuvering speed is the speed below which a sudden maximum elevator input will reach stalling AOA (thereby unloading the structure) before limit load is reached.  Turbulent Air Penetration Speed (TAPS) is the speed below which a theoretical worst case vertical gust will cause stalling AOA to be reached before limit load is reached.
As far as weather conditions, the biggest risk is flying into a thunderstorm cell and hitting extreme vertical gusts (mainly the up going air shaft in the center of the cell), and next to that would be Clear Air Turbulence which is usually related to crossing jet streams and passing through the shearing turbulence sometimes caused at the boundary between the jet stream and the adjacent air mass (jet streams reside in the little corner at the very top of a frontal boundary, in a little triangle of space at the top edge of the warm air mass, next to the adjacent cold air mass and the stratoshere above).  Another one is crossing rotors (spinning horizontal shafts of air) that reside below the crests of mountain waves that form downwind of mountain ranges at high altitudes.
For that eventuality you have to slow to TAPS any time you expect to hit severe turbulence like picking your way around thunderstorm cells or flying into areas where Clear Air Turbulence is reported.  In the absence of a CAT report, a cautious capt may slow to TAPS anyway if the crew notices that they are passing across a jet stream, just in case (you know you are crossing a jet when your ground speed suddenly changes and the autopilot starts making a sudden heading change to stay on track - it all goes back to normal when you leave the jet).
In theory, staying below TAPS provides protection against permanently bending the wings same as staying below MS protects from bending the wings from a full on elevator input.  If the 777 in the incident hit 3 Gs, it suggests that they were above TAPS when it happened but even if it was fully loaded, with the fudge factor in the structural design it's not surprising it wasn't bent from an encounter only half a G above limit load.

So was the Air India 787 really close to disintegrating in flight, since the turbulence caused accelerations of up to 3 G? Or is there another 50% factor on the 3.7 rating? (Since you said minimum). Thanks

Probably not because if the structure is designed to 2.5 limit load and 3.75 ultimate load, the worst case is some permanent deformation of the wings not breaking them with a 3G event.  I say minimum because while the cert requirement is 2.5G before permanent bending, the structural designers won't make it JUST strong enough for 2.5; there will be some extra to allow for "scatter factor" in the calculations, manufacturing and assembly flaws, ageing, damage tolerance etc.

You see that when at the end of a fatigue program in a wing test rig, where the manufacturer will bend the wing to failure just to see what it takes to snap it, and it lets go way beyond the design limit.

An analysis of commercial aircraft damage due to weather related wind shear may show side loading, rather than G forces, are the cause of excessive stress.  Looking at a = f/m, one can see a very large plane will almost behave as a fixed object in wind shear, and leveraging of forces presents the greatest danger.
This was the demise of many of the great airships in the 1920s, when they made a serious bid at transatlantic passenger and mail service and were considered for military applications as well.  Those unlucky enough to get caught in severe weather were literally torn in half by wind shear.  Their great size worked against them.
These incidents are much rarer in modern times, as faster flying jets are much stronger. But designers should be aware of the weaker points,  and pilots should avoid weather conditions that may exceed them.


text:
How many Airbus planes have been destroyed in flight by severe turbulence?
Since a brand new A350 has literally the exact same design specifications / G limits as an airliner built in the fifties does (A 1955 F27 Fellowship will break up at 2.5 G-3.75G, same as a modern Airbus jet - anymore than 2.5 G it will start to break up), and there were quite a few airliners from the 50's and 60's era losing wings or being destroyed completely as a result of turbulence.
Are there any Airbus planes that have broke up due to turbulence or accidents that were likely due to turbulence, but covered up?
Do Airbus planes have systems to avoid Clear Air Turbulence, and avoid flying over mountains, since most turbulence related inflight breakups occurred due to mountain wave activity? (BOAC Flight 911 for instance was flying over the top of a mountain - it experienced G forces of +9 and -4, and broke apart and crashed).
https://en.wikipedia.org/wiki/Wien_Consolidated_Airlines_Flight_55
https://en.wikipedia.org/wiki/BOAC_Flight_911
https://en.wikipedia.org/wiki/Braniff_Flight_250
https://ral.ucar.edu/aap/turbulence
(scroll down the last link for photos of the DC-8 jet with half its wing and engine torn off due to turbulence while flying at 31,000 feet over the Colorado mountains)

Why Airbus specifically?

Because  I already know Boeing's and MacDonell Douglas planes have. I could ask how many 737's have broken up in turbulence.

Your first sentence is just false. The certification specification have changed significantly in the fifties.

Hello Willy A. Asking for which accidents are due to turbulence but covered up is asking for pure speculation / opinion. We don't do that here. Therefore I vote to close this question.

@DeltaLima. How can you state a question is speculation, and vote to close it, and also submit an answer!! That is so hypocritical...!!?  The question is NOT speculation, because it’s based on a false premise, as your answer demonstrates.

"Are there any Airbus [...] accidents that were likely due to turbulence, but covered up?" I read that as "are there security agency reports that are based on falsification?"

@Penguin, thank you for your view on this. I see how that comes as hypocritical. The order events was that I read half of the question, then wrote a half answer to explain the false premise. Then I realised the real question wasn't answered with that, because the false premise was simply providing the context for speculation. I decided to VtC for speculation. At the same I also wanted explain the false premise, because if the root for the speculation isn't addressed, we will likely end up with many follow up questions of similar nature as we have seen in other cases in the past.

The Aviation Safety database lists 91 accidents due to turbulence as of writing. None are on Airbus aircraft. The most recent in flight breakup of a transport aircraft listed was in 1993.
All of the aircraft in your examples are fairly old designs, dating back to the 60's or earlier. Your statement that the A350 has the exact same design requirements as an F-27 is inaccurate. While the load factors required for certification may have remained constant, the actual loads aircraft must be designed to survive have changed over time. As an example, for transport aircraft, the FAA regulation on gust and turbulence loads is 14 CFR 25.341, which has been updated 3 times since it was originally introduced in 1965. For the F-27 incident in Alaska, one of the factors noted was cracks in the wings from a lack of maintenance. Maintenance has been improved since then, as has the durability of aircraft designs. As accidents happen and the industry learns from experience, designs improve to better anticipate the conditions that aircraft will actually experience.
Other than the DC-8 in your last link, all of the occurrences happened in the 60's. Apart from the design of the aircraft, a lot has changed operationally since then. Weather radars have improved dramatically, both on the ground and on the aircraft. Modern radars have a much better ability to detect turbulence, even clear air turbulence to a degree. They have also become much less prone to operator error. Weather satellites now provide information on weather even in remote areas. Forecasts have improved as well, providing pilots and dispatchers a better idea of areas that should be avoided.
Below is an example of the kind of information that modern radar systems can provide to pilots about weather conditions in their path.

Source

Thanks. I recently came across an incident report on a SouthWest 737 that encountered severe clear air turbulence. The FDR registered peak vertical accelerations of +3.52 G. Here's the report - https://www.aeroinside.com/item/3306/southwest-b737-near-oakland-on-dec-1st-2011-turbulence-injures-three-cabin-crew The Design Ultimate Load of a 737-800 is around 3.7G. Does this mean that 737 came really close to breaking apart? I'm interested in an answer to this, thanks!

@WillyA the report lists the aircraft damage as "none" and it actually appears to be on a flight right now, so I would guess it wouldn't have substantially exceeded limit.

If you've seen the 777 wing testing video, you can see how things go from ''a little iffy'' to ''oops wing snapped off''. I've looked at various websites about modern commercial airplanes and it says Ultimate Load is 3.7G..Ultimate Load is where severe structural failure will occur, eg wings snapping off, horizontal tail snapping off, etc.

Reading the BOAC 911 accident description I also noticed that “at an indicated airspeed of 320 to 370 knots”. That's rather fast for today's standards, and because the wings produce much more lift before stalling at higher speed it means turbulence caused significantly higher loads for them than if they were flying around 250–270 knots that would be typical climbing speed these days.

@WillyA everything above the limit load will cause permanent damage, and the video you referenced is testing to 150% of limit load, so there is a lot of room in there where the wing will not break but it has been damaged.

TLDR: As near as I can tell, probably none with the past few decades
I went to the NTSB's accident database https://www.ntsb.gov/_layouts/ntsb.aviation/index.aspx
This contains information since 1962.  It does contain information from foreign sources, although I do not know if 100% of foreign accidents are reported here.
I searched for Make:Airbus, Damage:Destroyed, Injury Severity:Fatal, and keyword "turbulence" within the summary.
There was only one result, American Airlines flight 587 November 12, 2001. However, if you read the report, you'll see that the turbulence was not severe (0.3g versus the 2.5+ mentioned in the question). The summary text said

The National Transportation Safety Board determines the probable
  cause(s) of this accident to be: the in-flight separation of the
  vertical stabilizer as a result of the loads beyond ultimate design
  that were created by the first officer's unnecessary and excessive
  rudder pedal inputs.

There was no reason for the FO to be seesawing the rudder during that incident. In a swept wing jet you never touch the rudder pedals once you are airborne unless there is an engine failure or some other asymmetric drag event because if you make big rudder inputs you get dutch roll started.  I would put my feet on the floor once we were above 1000 ft or so because if I left them on the pedals I would instinctively squeeze bits of rudder when applying aileron, and you're not supposed to.  The yaw damper takes care of that stuff by itself.

Aircraft design specification have changed significantly since the fifties of the last century. Your claim that the:

a brand new A350 has literally the exact same design specifications / G limits as an airliner built in the fifties does

is just false.
See for yourself at the FAA website. For large aeroplanes, the applicable regulation is FAR 25.

Then why do both the old 707 and the new (say, A350) have Design Ultimate Load @3.75G? Past Design Ultimate Load, then things like wings come ripping off at the roots. If modern aircraft were stronger in a way they could have survived the turbulence that tore the older era aircraft apart, wouldn't the Design Ultimate Load be something like 5 or 6G? Why is it the same as vintage aircraft? This is something I don't understand so I'd highly appreciate it if you could clear this up for me.

That is an excellent question! I am happy to answer that if you ask it as a separate question. It might take a while, as I am about to leave for a trip, but I'll answer it eventually.

Ah okay. I'd be very interested in an answer!

@WillyA - that would actually make a much better question. Also, While DeltaLima is good (really good!) he's not the only one who can answer questions, so don't worry about the fact that he won't be around for a while.

@FreeMan thank you for those nice words. WillyA, FreeMan is right, there are many members around here who will be able to give you an answer better than I would be able to give, so please don't wait for me.


text:
Where do extra planes come from?
Last night, I had to change planes at LAX because the United Airlines B737 that I was flying on had radio problems and thus could not fly over the ocean. One of the hi-frequency radios had problems, and the pilot said that both radios must function in order for the plane to fly over the ocean. So I flew on another B737 and arrived at Honolulu almost two hours late.
Do airlines keep "spare" planes at major airports in case problems with planes that are scheduled to fly happen? What is the procedure for getting a replacement plane? A plane could not be flown in at a moment's notice, obviously, because it could take hours for it to arrive.

Was LAX an expected stop in the plane's itinerary? In other words, was the flight already scheduled to land at LAX, or was LAX chosen as an unscheduled stop in order to permit changing to a different plane? The former implies the replacement was found/made to be at the location. The latter would give the airline multiple possible airports at which to find a plane to swap out, without the need to move the replacement. So, what was the original itinerary?

As far as I know, the plane with the malfunctioning radio was going to fly from LAX to HNL. That plane was parked at gate 84. Its replacement was parked next to it at gate 83 when it arrived. When I boarded the replacement, I saw that the plane I originally boarded already left the gate, possibly to a maintenance hanger.

That doesn't make it clear, but implies that LAX was an expected stop on your trip from wherever you started to HNL. Was the original expectation that you would travel from somewhere➞LAX➞HNL all on the same plane? Or, were you initially expecting to go from somewhere➞HNL?

If you have a flight number I think that may help a bit.

UA1232 was my flight on November 9 from LAX to HNL.

Two big airplanes really like each other, and then ... ask your dad.

"both radios must function in order for the plane to fly over the ocean"  So it can still fly over land.  Seems like the solution would be to just switch an aircraft that has it's radios working that is land locked with this one until it's repaired.

Not a real answer but may explain a bit: Once I was flying from the UK, we had to switch planes for similar technical reasons. In the captain's address, he said "we are going to be about twenty minutes late, which happen when you take someone else's plane".

@Harper - came here to say that. Leaving satisfied. :-D

This is touched on a bit in this travel.se question. Strictly speaking they don't really keep "spares" per se. It's just too expensive.
There are a lot of factors to this and flight planning has gotten much better over the past few decades so planes are rarely flown empty, moved for no reason, or just sitting around any more like they did in the past. The prevailing mentality is that an aircraft is only making money when it's flying, so keep it in the air as much as possible.
However the reality of airline operations requires aircraft to come in and out of service every so often for maintenance and routine checks. This creates a bit of wiggle room in the fleet, so while there are no spare aircraft per se, a fleet of sufficient size may have excess resources. The chances of getting a replacement aircraft are greatly increased if you are at an airport the airline considers a hub or an airport that has large maintenance facilities. Ultimately the demand is not always there to keep all planes in the air all the time so spare capacity is sometime driven by the nature of the business.
In your particular case it's also possible another aircraft was swapped in with the assumption the HF would be fixed fairly fast and thus capable of being dispatched for a flight in fairly short order filling the void it created.

A plane could not be flown in at a moment's notice, obviously, because
  it could take hours for it to arrive.

That depends on where the other plane is sitting. Sometimes an aircraft can be flown in if it's sitting at a nearby airfield.

These days it's often cheaper for an airline to cancel the flight, put everyone up in a hotel and book them on the next available flights to the destination. This will obviously be weighed by the airline prior to doing so, but it's a risk they know they have and one they are typically fairly prepared for.

The only operation that I know of, that keeps a full spare on hand 100% of the time, every time, is Air Force One.

Or the aircraft with the defective HF radio could be swapped with one that was planned to do say LAX to Denver, where those radios wouldn't be required.

@jamesqf i figured that was the reg but i was not 100% sure so i did not include it in the answer. I can always update.

An aircraft is only making money when it's flying with paying passengers in it...

HF radios are only used on transoceanic flights, so in that specific case, all the airline had to do was swap your plane for another one at LAX that had a working one but didn't need it, i.e. a continental flight. Ideally, that plane would be on the ground at LAX long enough for them to replace the radio there, but if not, they could prearrange to have it replaced later somewhere else, probably overnight.
More generally, as others have noted, airlines don't really keep spare planes around. However, many of the planes out of service for maintenance could be pressed into service if needed without too much hassle, especially at hubs like LAX, so there is some slack in practice.

Exactly. All UA had to do in this particular case was swap equipment with a different flight if they didn't have any aircraft sitting around that weren't scheduled to go anywhere soon. Hawaii is pretty much the only type of flight from LAX on a 737 that would require HF radios. They could potentially go anywhere in the CONUS without them.

My experience is on the Regional side but in that business spare aircraft are common with operators that have large fleets.  There are two important metrics: Dispatch Reliability and Schedule Completion Rate.
DR is % of aircraft leaving the gate within 15 minutes.  The industry standard is 99% or better, meaning airplanes leave with a delay of over 15 minutes not more than once every 100 departures.
SCR is flights that were completed one way or another, as opposed to being cancelled.  SCR should be above 99.5%.
An airline can have a fleet with at DR that is below 99%, sometimes well below, but by having spares they can achieve SCRs close to 100%, which is the critical metric overall.  In many cases, especially if the fleet is large and well ammortized, it's worth the cost to keep spares.


text:
Why are tugs the least priority when talking to ground?
I was listening to this ATC conversation (3:10) and wondering:

Why are tugs the least priority when talking to ground? I am thinking that some tugs might be towing aircraft that need to depart.
Why are tugs even talking to ground? Whenever I am on a plane at some airports, I see roads next to taxiways with cars, buses, and other vehicles that stop when airplanes are about to cross their paths (such as the roads with the dashed white lines in the picture). Do tugs travel on roadways or on taxiways? Do tugs communicate with ground while they are on a roadway?

1) The video you are specifically referencing is JFK (as you probably know). JFK has a very complex airport layout and generally is very busy. There are a variety of reasons "tugs are last priority."
While it's not a specific rule to always allow airplanes under their own power to move first - Air Traffic Control exists to expedite the flow of traffic, which includes ground traffic, and often times tugs are just slower and because they are often relocating to different gates, pads, or hangars, they tend to get in the way of the normal flow of traffic.
Often times departing airplanes need a specific sequence (order) at the departure runway which cannot be delayed by lower-priority things such as airplanes being tugged around the airport.
Airplanes that are being tugged around are not under their own power and not burning nearly the amount of fuel that a taxiing aircraft is. Again, specifically with JFK, there are so many airplanes the ground controller must keep things moving to have an even somewhat organized and expedited flow. Often times, tugs are moving airplanes that are not filled with passengers, so the theory being "they can wait" (needs of many outweighs the needs of the few).
Even more often, and specifically with JFK, the airlines schedule causes those "tugged" airplanes to want to come into a one-way "alleyway" in the ramp when many revenue passenger filled planes are trying to exit. The tug with the airplane gets to wait, because they physically can't get into the alleyway anyways until all the airplanes taxiing move.
2) In the US, airports have what are called movement areas and non-movement areas. Movement areas are generally controlled areas, by either ATC "ground control" or by a "ramp control." Non-movement areas, contrary to its name, are areas that are not controlled by ATC or a ramp control and up to the user to maneuver without control instructions. Tugs only communicate with ground when they are on movement areas (taxiways and runways). Roads are generally non-movement areas. At some airports (JFK being one of them) when a road intersects a movement area, a vehicle on the non-movement area must always give way to traffic on the movement area. So, a tug driving around without an airplane attached would be on the road most likely, and only talk to ground control if that specific airport required a clearance to "Cross" the taxiway at the road. This is commonly referred to as a "look and go" operation. Look and go is specifically authorized only for roads that intersect taxiways at certain airports, and never allowed for runway crossings. Busses almost never operate on taxiways, except to cross them as described above. In general, unless a vehicle has a specific purpose to use the movement area, they'll use a road, because it's easier.


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Is there a specific procedure for grounding a pilot?
When a pilot gets grounded by the FAA, is there a specific procedure that happens?
I have heard of situations where the pilot is recommended or told to ground themselves. Is this the same as if the FAA were to do it or does the pilot just say "I'm not going to fly today"?

This site https://thepilotlawyer.com/faa-emergency-orders/ talks about emergency revocations. Bob Hoover was subject to one fo these that was eventually overturned. “The FAA has the authority to suspend or revoke an airman’s certificates whenever the Administrator determines that the public interest and safety in air transportation or air commerce requires so.”

It depends on how/why you are grounded and who does the grounding.
The FAA can legally ground you if your license gets suspended (or in rare cases completely revoked) there are formal proceedings the FAA has in place. You can find the AC that covers it here.
AME's can can deny you a medical certificate which can prevent you from flying. In this case you are also legally grounded as you may no longer comply with FAA regulations.
AME's may also make grounding recommendations. Say you start taking a new medication that is legal on the FAA's list. The general advisory is to "see how the medication effects you" prior to getting back to flying. Often this is subjective and there is not really any hard guidance on time limits for every medication. These groundings should be self imposed by the pilot.
You can be practically grounded by say a flight school or insurance company. This falls more into the "not fit to fly" category. Technically you are legal to fly but other practical limitations may prevent you from flying. Lets say you show up to rent a plane and you are clearly sick. You say you "feel fine" but the flight school is under no obligation to rent to you and may chose not to if they feel you are not fit to fly.
Every pilot is responsible for a self assessment prior to making any flight to ensure they are fit to fly. Ultimately an infraction could fall under 91.13 Careless or Reckless Operation. In this case you may or may not be legal to fly but its up to you to make that self assessment and not fly.

It depends what you mean by "grounded".  In the case of suspending or revoking a license, I think it requires a civil case in front of an Administrative Law Judge--unless the pilot voluntarily surrendered it, of course.  This would most likely happen after there was an accident or incident investigated by the NTSB, or possibly a serious deviation reported by ATC (and not self-reported to ASRS).
Less formally, if an employer, fellow crew member, instructor, etc. has serious doubts about whether a pilot is fit to fly, then given the risk-averse nature of the field, it's expected that the pilot would voluntarily ground himself as a precaution until he could resolve those concerns.
FWIW, I ran across this recording where ATC declared a pilot a "risk to aviation safety" and ordered her to land immediately--literally grounding her.  I suspect FSDO took a serious interest in that afterward, but I don't know what the final result was.

There is a number of factors and situations that can trigger a FAA hearing or investigarion that can lead to revocation of your pilot certificate. Among them, failed ramp checks, DUI issues, medical compliance, and so on. You should take these folks seriously, as it's much easier to stay off their regulatory radar then to get off their black lists.
Another reason for grounding a pilot could be determination of you being "unfit for flight" which is a broad term for many kinds of psychololical and emotional impairments that make you inconsistent with general and specific safety regulations.


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What is this? Multiple small cracks in passenger window
I have seen this many times in typically older aircraft passenger windows and on a recent flight from Sydney to Melbourne I took the following pictures. It appears as if there are small cracks in the window or one of the laminate layers used to construct the windows. Could anyone shed some light on what is the cause and if there is any risk?

Airline pax windows are cast acrylic plastic (plexiglass) and when the panels age they can get little microcracks in the surface called crazing.  This can be formed from long term tension stress on the crazed surface as well as from degradation from chemicals (ammonia is one of the worst - NEVER use Windex or equivalent on plexiglass) or UV (acrylic cast for aircraft has a UV block in it that makes it relatively resistant to UV).
On airline windows it's mostly due to long term surface stress because the pressurization system is trying to blow the window out all the time and the outer layer is in tension.  Minor crazing is not a structural problem because it is in the outer couple thousandths of an inch of the plastic and doesn't reduce its strength significantly (1/4" thick acrylic sheet at 8ksi tensile strength, like you would typically find in a pressurized aircraft window, is roughly equivalent in tensile strength to .050" 2024 aluminum, so it's not that much different than the adjacent aluminum skin with minor abrasion surface scratches, structurally speaking).
Airlines restore windows with crazing and scratches by removing the outer crazed layer with abrasive products made for the purpose (like Micro Mesh https://micro-surface.com/).  There is an allowable minimum thickness of the acrylic and they can go down to that, and if crazing is still apparent at that point the window is scrap.

Thanks John, That makes sense and I appreciate the time and effort providing such a detailed answer. Cheers Ian

I was in China on business once and rode on a Shandong Airlines 737 with cabin windows that appeared to have been restored using Micro Mesh (a very special type of rubberized sandpaper) and they didn't do a particularly even job of it. My window was wavy like a window pane from a 19th century farmhouse lol.

There is usually a plexiglass cover on the inside of an aircraft window. There tends to be a little hole in the plexiglass cover to make sure there is not much pressure difference on the cover. The cracks are probably only on the cover, and not in the window itself. If it really is the window, it was probably replaced already. The cover has no structural role, it is just for thermal isolation, and to keep passengers away from the real window, so it is nothing to be worried about.

This seems unlikely as there is no stress on the inner window due to the vent hole you mentioned. John K's answer seems much more likely.

These cracks are much deeper than a few thousands of an inch, you need a magnifying glass or a microscope to see cracks that small. Constant stress is not needed to cause cracks, the inner panes are loaded during (de-)pressurisation.

There is a bit of an optical illusion effect that makes crazing look much deeper than it really is.

I sure hope so, because it looks like they are about 2 mm deep. Bending resistance decreases with the thickness to the power 3, and on top of that you get a stress concentration factor due to the crack. So cracks can be a pretty big threat to a window.


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How the aircraft sense the sink rate during autoland?
Radio altimeter is required for autoland, and the target of autoland flare mode is to reduce the sink rate to an acceptable sink rate at touch down， then how the sink rate is computed by the radio altimeter since radio altimeter just provides the height information rather than the rate?

The radar altimeter doesn't calculate the rate of descent. A radar altimeter only gives height information, but it can take many readings very quickly and feed that data to a computer which will calculate the rate of descent.

VS data from what @ymb1? VS data can be calculated from the radar altimeter.

It depends on how accurate you want to be @ymb1, air data would be okay for approach, but when you're 20 feet off the runway small errors can make for firm landings.

The problem is static ports misread in ground effect, so rad alt is more useful close to the ground.

Not only that; the air VS data has a significant delay, which is the last thing you need during flare. Essentially, the VSI (or ADC) also differentiate the altitude (or air pressure) to get VS, like RA could, even if mechanically.


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What is the relationship between ADS-B IN/OUT and different mode transponders？
What is the relationship between ADS-B IN/OUT system and different mode transponders? Can a aircraft with mode A or C transponder have the ADS-B in/out capability, that is getting more detailed surrounding traffic information like ID, position, velocity and broadcasting its own ID, position, altitude etc.

While it is normally integrated into a single transponder, there is no requirement to have a Mode A/C transponder to be able to also have an ADS-B in/out capability. I'm not sure what you are asking? Mode A/C transponders are not ADS-B capable by themselves, ADS-B is a separate protocol that has to be specifically supported. Are you asking about in-cockpit displays? ADS-B in display depends on the other avionics you have in the aircraft...

@Ron Beyer thanks, Ron. So mode A/C transponder can not be a component for a ADS-B IN/Out system. What onboard equipment can get the traffic data thru the mode A/C transponder equipped on another aircraft?

And what onboard equipment required for ADS-B in function to get the data broadcasted by the mode-S transponder on another aircraft?

TCAS is the only system I know of that allows one aircraft to show data from another through mode a/c transponders.


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How to reduce the landing roll distance
If I want reduce the landing roll distance, for example to clear the runway at the nearest exit, I was told by some instructors that to increase the drag using following methods:

Pull up and hold the elevator at the upper extreme position.

Question 1: does that mean I should try to keep the nose wheel in air as long as possible?Is there any risk I leave the ground again due to aggressive pitch? Will there be any effect after all the wheels touched the ground, like keep the elevator up can increase the friction of the main wheels?

Retract flap to reduce the lift to create more frictions.

Questions 2: when flap is retracted, I will have less lift so more friction force could be induced but does that mean the aircraft is in a cleaner config, so the drag is also reduced?

see aero braking https://aviation.stackexchange.com/questions/51834/is-aerodynamic-braking-really-effective/51835#51835

My experience shows that full flap deflection (the opposite of retracted) increase drag dramatically, especially on short final. Spoilers may be best suited to decrease lift.

You didn't specify what aircraft you are talking to but brakes are the most effective way to stop in the shortest distance.

@wbeard52: Not always true.  Brakes aren't nearly as effective on dirt or grass (especially grass wet from dew or recent rain) as they are on pavement.  And they're less effective on wet or snowy/icy pavement than dry.

1) This does mean you should keep the nose up as long as possible, increasing the angle of attack when you are behind the drag curve increases the drag the wing produces and uses the elevator as a spoiler in effect. If you have done a full stall landing pulling up cannot get you back into the air, but if you land with extra speed pulling up could mean you start flying again, so you should only use this technique if you are sure you have a low enough airspeed. Good technique is not to yank the stick back, you pull it back while paying attention to the "feel" of the airplane, if it seems to want to fly again ease off on the back pressure.
2) Retracting flap after landing decreases lift and puts more weight on the wheels, making the wheel brakes more effective. It does decrease drag, however your wheel brakes are much more effective at slowing the airplane than drag from the flaps.

You didn't specify what type of aircraft you are asking about.
I will tell what I learned during my micro-light flight training (probably partially applicable to smaller General Aviation planes aswell).
For a tricycle landing gears you are supposed to land with the nosewheel up, since the landing gear is quite "flimsy" on micro-lights.
So breaking while the nose is up introduces a forward momentum, which can bring your nose down quite hard, risking damage to your nosewheel and a prop strike.
After flaring out you keep the elevator pulled usually but at some point the nosewheel comes down by itself and you can break safely.
In STOL competitions you see many taildraggers hitting the breaks hard, as soon as they touch down. There the breaking has to be controlled very well, to not get a prop strike.
For airliners in general it is a bit different (massive landing gear, good runway condition). I think as soon as the spoilers come up the airflow is disrupted and the lift component of the flaps is reduced (negligible).
There was one famous landing of a stripped down IL-62 (called "Lady Agnes") landing on a 900m grass field, where right after touching down with all three wheels, they got the nose back up to use the whole wing as an airbrake and the extended spoilers to keep the main landing gear down for breaking. The flaps staid extended during the whole landing and rollout.


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How to do the land maneuver if I got complete thrust loss after liftoff with runway ahead?
I am flying a single engine aircraft and after liftoff in air at low height, I lost the engine but I till have some runway length ahead, what should I do? Land straight ahead? How could I tell if I still have enough runway length to do the land maneuver and full stop on the runway? How should I perform the landing maneuver, quickly idle thrust and pitch down to stop the climb rate and then do a pitch up flare?

At low altitude engine loss you have no choice but to land straight ahead. Anything but the slightest heading adjustments will most probably end catastrofically. Your memory items are (at least, depends on complexity of aircraft):

Gently but decisevily lower the nose
Close throttle
Mixture cutoff
Close fuel valve
Adjust airspeed for landing
Flare and land
Now is the time to worry whether or not you have room to stop. Brake in a controlled manner and avoid objects if possible.

Should you have the time and composure at any moment during the predicament, radio "mayday mayday mayday [your callsign] engine out"

Try to crack your door open before hitting anything.

How do you determine if you have enough runway ahead? That's a tricky one, and in this situation you quite possibly have milliseconds to make that determination. It's going to be an estimate at best because you certainly wont have time to do any calculations. As a general rule I'd say if you have around half the runway length left you're probably good.
In this situation you should definately make a straight ahead landing, even if you dont have enough runway left.
Cut the throttle (if you have any left, in say a partial failure/rough running), pitch down to landing attitude, and execute a normal glide approach landing.

"As a general rule I'd say if you have around half the runway length left you're probably good."  That depends entirely on the overall length of the runway!

If you've lost power, you really don't have a choice.  Your best bet (probably your only bet) is to use whatever runway you have to slow as much as possible before crashing into whatever's at the end.

The only time I can think where you might consider not landing straight ahead is when you are taking off from a short runway and your power fails as you are over a longer cross runway. Making a turn to land on the longer runway might be advisable, but even then you risk stalling in the turn and converting a perfectly safe landing into a crash.

@DJClayworth: Or perhaps a parallel runway, if there's no traffic using it.  E.g. my often home airport used to have an (approximately) 5000 ft GA runway paralleling the 10,000 ft commercial one, so there was a couple of thousand feet of the long runway extending beyond each end of the shorter one.


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What are the disadvantages for using short-field take-off technique?
It is obvious that using the short runway takeoff method(hold the brake till engine get full power) could get the aircraft in air within a shorter runway distance.Then why it is not used for every takeoff for all different runways? My understanding is that occupying less runway should always be a better option, isn't it? What are the disadvantages for using short runway takeoff method?

Related: Could an aircraft be tied down, then throttle up and release for takeoff in order to use a shorter runway?

A rolling take off may also save some runway as you star using the runway with some speed you don't need to acquire at the beginning of the take off role.

In my RV-8 if I apply full power with brakes, I'll get a prop strike.

Three main things:

There is a little bit of increased brake wear because you apply them to stop, they usually creep a bit at full power, and when releasing them, there is also that short period where they are slipping while under pressure.  More wear than not using them at all anyway.  Of the three this is pretty trivial however.
Sitting stationary at full throttle has the engine at maximum heating rate and minimum cooling flow.  The area of the exhaust valve on the head heats up very quickly.  During maintenance, full power engine runs except for very short bursts are to be avoided without a special cooling box shroud, to catch large amounts of air and force it down between the cylinders, installed in place of the cowling.
The full power condition while sitting stationary turns the prop into a pretty powerful vacuum cleaner and it will suck up just about every pebble and bit of grit on the pavement (and even clean looking pavement is full of that sort of FOD).  Or water if you are over a puddle (and water erodes aluminum).  It will leave you wondering why you have to constantly have your propeller dressed out to clean up all the little stone nicks (this is why I always do run-ups over a patch of grass whenever possible).  Of the three, this is probably the biggest item in my experience.

Good points. I had forgotten 2 & 3 which are both significant.

Sliding slightly OT: As I understand it, the grass is usually left a bit longer at airports and (potentially) lots of debris could blow in and get stuck in longer grass. Is there not a worry about sucking things up out of the grass?

The grass blades inhibit the boundary layer of air being sucked from all directions into the little tornado that forms below the prop, so that objects are a lot harder to be pulled up.  Like vacuuming a ball bearing sitting on a flat surface vs one at the base of the bristles of a brush. You can hold the nozzle farther away on the flat surface.  I operate from a strip with a tar/gravel runway but I run up on the grass and start the TO roll on the grass before the edge, and I get very few propeller dings and the ones I get are tiny.

@John K: A better example might be the difference between vacuuming a hardwood floor vs a carpet.  Carpet traps a lot of dirt, which is why vacuums often have a special carpet-beater attachment.

Yeah that works too.  The other thing is that the base of grass blades is mostly soil so you have way less of the nasty objects like pebbles or small chunks of limestone in the first place.

You'll also burn more fuel, because you're sitting there running the engine without going anywhere.

What are the disadvantages for using short runway takeoff method?

About the most tangible I can think is additional brake wear. However that  is negligible, just holding brakes isn't going to wear them that much faster.
What I actually dispute is this:

[...] occupying less runway should always be a better option

I would ask yourself why you think this. More runway gives you more time to

scan engine instruments
Check airspeed increasing as appropriate

Those 2 things are the primary checks on the take off roll. By rushing to get off the ground you might miss something that with an extra second or 2 you might have otherwise caught.
When genuinely attempting to take off from a short field, we might sacrifice an acceptable level of safety margin, but as a general rule on a mile long stretch of tarmac - it's just not worth it!


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Temperature sensor are needed to compute True Airspeed for low speed aircrafts?
I know standard air data computer can produce true air speed with total air pressure, static air pressure, and total air temperature. But for a low speed aircraft <100 knots, do I need to install a total air temperature sensor or other temperature sensor to get the true air speed computed? Or air temperature is not needed for low speed aircrafts and the air data computer can produce the true airspeed based on the calibrated air speed?

A good chunk of aircraft operating in the <100Kt range generally don't have Air Data Computers although glass cockpits are becoming more and more common in general aviation aircraft. For good old steam gauges a sliding temperature compensation scale is built right into the Air Speed Indicator, for more info on that see this question or this video. OAT is determined generally by a thermometer installed on the windscreen or in the side window.
If your aircraft is equipped with with a capable glass cockpit you may be able to get TAS from the system

The Airspeed Indicator displays airspeed on a rolling number gauge
  using a moving tape. The true airspeed (TAS) is displayed in knots
  below the Airspeed Indicator.

The G1000 has an OAT gauge as part of the install.

The Outside Air Temperature (OAT) and the deviation from International
  Standard Atmosphere (ISA) temperature are displayed in degrees Celsius
  (°C) by default in the lower left of the PFD under normal display
  conditions, or below the groundspeed in reversionary mode.

So for a low speed aircraft, in theory TAT is not needed for TAS computation, but the OAT(or SAT) are required? Could I got more elaboration on the computation method, for the TAS computation at low speed?

@YoungL none of the small planes i fly have a TAT probe they all rely on OAT (see links in body for more info on how to use OAT for corrections). As for the computation in an air data computer I don't know exactly how Garmin (or any one else) does it only that it is done.

@YoungL, actually it seems it is always OAT that is needed. The difference is negligible at slow speeds anyway.


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Autothrottle's control target during autoland
I read some pilots manual which indicates that the AT retard mode will be active at 30feet AGL to retract the throttle to idle, but the flare mode starts from 50 feet AGL? Should the throttle to be retracted to idle before flare? If not what is auto throttle doing during the flare maneuver, try to maintain the speed?

The auto-throttle system of a typical airliner (like e.g. the Boeing 737) will maintain the approach speed (usually $ v_\mathrm{ref} + 5 \, \mathrm{kt} $, depends on the wind) during final approach. It should not reduce thrust before starting the flare because the speed could decrease too much, which could in the worst case result in a stall.
The Boeing 737 FCOM (v2 4.20.19 Automatic Flight - System Description, see also this answer for a full description of the autoland) says:

The A/P flare maneuver starts at approximately 50 feet RA and is completed at touchdown. [...]
The A/T begins retarding thrust at approximately 27 feet RA so as to reach idle at touchdown. A/T FMA annunciates RETARD.

So during the flare the thrust is decreased such that the thrust levers are closed at touchdown. This (together with the increase in pitch) will start to reduce airspeed. Ideally, the aircraft then touches down at or about reference speed $ v_\mathrm{ref} $. The thrust reduction should be smooth because the thrust-pitch-coupling of engines mounted under the wing will cause a pitch down moment, which needs to be counteracted with control inputs.

Thanks for the clarifications, so ideally autoland flare at 50 feet with airspeed Vref+5, touch down at Vref, does that mean(ideally) I always land with same pitch attitude at touch down?

Not necessarily, the pitch attitude will depend on the weight of the aircraft and the configuration (737 typically lands with flaps 30 or 40) and to some lesser extent atmospheric conditions (air pressure, temperature).

Airbus is very similar.
During the approach A/THR is in SPEED mode, targeting the pre-determined approach speed, usually VLS+5 kt. During the flare, if autopilot is engaged in LAND mode A/THR will automatically reduce thrust to idle. Excerpt from FCOM:

RETARD MODE
The RETARD mode is only available during automatic landing (AP engaged in LAND mode). At approximately 40 feet RA, the RETARD mode engages and remains engaged after touchdown. The A/THR commands IDLE thrust during the flare, and the FMA and engine warning display “IDLE”. If the autopilot is disengaged during the flare before touchdown, the SPEED mode replaces the RETARD mode, and the pilot has to manually reduce thrust.
Note: In an automatic landing, the system generates a “RETARD” callout at 10 ft radioaltitude (RA), which prompts the pilot to move the thrust levers to IDLE in order to confirm thrust reduction. In manual landing conditions, the system generates this callout at 20 ft RA, as a reminder.

Don't be confused about "remains engaged after touchdown" phrase above. In practice pulling thrust levers back to idle at 10 ft as suggested by RETARD auto callout will disconnect the auto thrust just before touchdown.


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What is the standard report when pilots exiting and clearing the runway
Let us say I landed on runway 7 and will exit the runway thru taxiway Charlie, is that a standard report like "(Aircraft ID) taxing clear RW7 at Charlie"?

Are you asking about an uncontrolled airport?

What country are you asking about?

@Bianfable for both controlled and uncontrolled

@DanHulme how about in US?

Related: Why do pilots say “clear of the active” at non-towered airports?

Nitpick: that would be runway 07, not runway 7
The phraseology is mostly standardised, and in general consists of notifying the relevant unit that you have vacated the runway. At a small, GA airfield with AFIS in the UK the typical communication goes as follows.

ATCO: G-ABCD Cleared to land Runway 07, wind 240 at 5
  G-ABCD: Cleared to land, Runway 07. G-ABCD
  ... controlled crash ....
  G-ABCD: Runway vacated at Charlie

This becomes a bit more formal at controlled airfields. In the UK, Cap 413 is the radio telephony communications bible. Secion 4.89 "Runway Vacating and Communicating after Landing" is the part you'e interested in. The communications would be more along the lines of (Note: CAP413 uses the name "Kennington" as a fictitious airport)

Kennington Tower: BIGJET 347, vacate left
  BIGJET 347: Vacate left BIGJET 347
  Kennington Tower: BIGJET 347, when vacated contact Ground 118.350
  BIGJET 347: When vacated Ground 118.350, BIGJET 347
  BIGJET 347: Kennington Ground, BIGJET 347, runway vacated
  Kennington Ground: BIGJET 347, Kennington Ground, taxi to Stand 27 via taxiway Alpha

“Clear of active at Charlie”， what does Charlie stand for?

@VvV "Charlie" is "C" in the ICAO Phonetic Alphabet. Taxiways are often named sequentially, Alpha, Bravo, Charlie etc. So it means the aircraft has exited the runway at a specific taxiway that the controller knows (They may not be able to directly see the aircraft).

"Clear of active" does NOT seem like proper phraseology to me, since it would be extremely easy to confus the word "clear" with the word "cleared". Mentioning a runway in the same sentence is a recipe for disaster. The correct phraseology is "Runway vacated".

@J.Hougaard You're probably right. Bad habits prevail. Answer updated.

In your first example, why would ATC be clearing someone to land at an uncontrolled airport?

@J.Hougaard In the US, I always give the runway number as well, especially at an uncontrolled field, since there may be multiple active runways. So it would be, 'N1234 Clear of Rwy 07 at Charlie'. Or 'N 1234 Off Rwy 07 at Charlie'.

The FAA has published an Appendix to the Pilots Handbook of Aeronautical Knowledge that gives this example of exiting the runway.

Initial Contact After Landing and Clearing the Runway
Pilot: Lincoln ground, November 123QY, clear of Runway two at Bravo, taxi to the ramp.
Controller: November 123QY, Lincoln ground, taxi to the ramp via Bravo.

You still hear some pilots say “Clear of the active.” but it’s mostly older pilots who were trained a long time ago and are set in their ways.
By saying who you are, what runway you just exited, and where you are, and (if it’s not obvious) where you want to go, ATC can more easily get you off of the movement areas and to your final destination.
It works at uncontrolled airports as well since landing traffic knows which runway you exited. That’s especially useful if there are multiple runways in use. It also helps taxiing aircraft because they where you are and where you are going so they know if they need to hold for you to use the taxiway or if they can proceed to the runway.

In North America, it's "clear".
"Vacated" is used in UK and possibly other places that use UK CAA practices as a basis of their conventions and rules.
You will almost never use the phrase at a controlled airport; you pull off the active at the end of the landing roll, and when tower sees you are pulling off onto the taxiway, just tells you to contact ground with the frequency.
At uncontrolled airports in North America you will always hear "XXX is clear the active".

"Vacate" is standard ICAO phraseology used world wide, with the USA being one of the very few possible exceptions

And Canada.  I've been flying 40+ years and I've never heard the term "vacate".

"Vacate" vs "Clear" was a result of the collision at Tenerife, following which pilots do NOT use the word "Clear" for anything, only controllers can use it. Which then necessitated the invention of "Vacated" for "I'm not on it any more", as opposed to "It's safe to do this...".


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Which avoidance maneuver is best when seeing crossing traffic on approach to a runway?
Let us say I am in final landing and I saw another aircraft is crossing the runway while I am still in air, should go around be my best choice?
If I touched down and then I saw the traffic, how could I make the best decision to avoid the conflicting traffic, increase throttle to go around, or increase brake force to stop before the impact, or decease the speed and try to exit the runway at a high speed?

What type of aircraft and type of airfield are we talking about?

For both GA and commercial aircrafts, controlled airport and uncontrolled airport

How is this different to your last question?

@Jamiec While very similar, that question asked about takeoff, this one about landing.

So in last question, most folks prefer takeoff abortion for a crossing traffic, does that mean for landing if I touched down, braking is a better option than a touch and go?

As the answer states, not landing at all in that situation is the only choice.

When you are on short final and something is on, is entering, or even seems to be entering the runway, you execute a go around.
As for the second part of your question, given the loose parameters I'm afraid there is an infinite amount of different scenarios, and therefor I can't provide a comprehensive answer.


text:
Questions about go around safety
When I perform a go around, how it is guaranteed there will be no traffic on my go around route for both controlled or uncontrolled airport?

It's not. See and avoid applies, a lot of pilots side-step the runway so they can watch for traffic better.

[...] how it is guaranteed there will be no traffic on my go around route [...]?

Nothing is guaranteed, ATC will do its best (at controlled fields) to make sure that the spacing is such that going around will not make you run into traffic at least for as long as it takes for the tower to notice you are going around, and then issue instructions on how to enter the pattern again. But the final responsibility lies with you to see and avoid, even at controlled fields.
At untowered fields, it is your responsibility to rejoin the pattern in a safe manner. Usually this means not doing anything unexpected, like turning directly into the downwind or popping up into the pattern from below. It is your responsibility to "see and avoid". You may not be aware of all the traffic around untowered fields, even if you did get a traffic report on the way in. There are aircraft out there that are (legally) operating without transponders and even radios. The best tool a pilot has to avoid MAC (mid-air collision) accidents around an airport is your eyes. The second best tool is the adherence to procedures. Third are your ears, listen to position reports and build a mental picture of the airspace.
For IFR plans, this means following your missed approach procedures if you haven't already cancelled (or going up again and picking a new clearance). If you've already cancelled then you are VFR and VFR procedures would be followed.
The way I do go-arounds is:

Initiate the go-around procedure according to the POH
When positive rate/control is established, notify tower/CTAF
Side-step the runway to the opposite of the downwind side to increase visibility
Enter cross-wind 300 feet below pattern altitude if clear, or follow ATC instructions

At untowered fields some of the biggest things you need to watch out for are:

Aircraft in the pattern that are not reporting position
Aircraft entering the pattern unconventionally (extended downwind, extended final, etc)
Aircraft back-taxiing

At a controlled airport, the tower will route you or other aircraft as needed for seperation. If IFR, you will have a clearance to follow if you have to go around, either from ATC for that area, or as published, and once following the clearance you will go back to ATC for further information.  At a non-controlled airport and IFR, the same will apply. If you land, you call and cancel the clearance, if airborne and you are talking to ATC and have visual to land, you may be able to cancel while airborne.
VFR is similar - either tower will direct you as needed, or you see and avoid when non-towered.   If you use flight following, you will be talking to ATC until you get near the airport, they will often advise you of any nearby traffic before cutting you loose for your own approach and landing, with their last direction being something like "Nxxxx, airport is at your 12:00 and 8 mile, you have traffic at 1:00 and 5 miles, squawk 1200, frequency change approved."


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Can aileron control maintain track in a single engine failure on a twin?
Let us say I am flying a twin-engine aircraft and I got a engine failure during cruise, without any control on rudder and use the ailerons only, can I still maintain the target track or heading? Will I got any sideslip?

You would need a pretty significant roll to counter the yawing effect of the dead engine...

Without aileron you may not be able to maintain control, much less keep on your track.

@RonBeyer: According to this page, 8/10°.

Why are you asking this question? Is there a particular reason you don’t want to use rudder? What type of “twin-engine aircraft” are you asking about? There is a big difference between a light piston twin with a 160hp versus a B777 with 115,000 lbs of thrust on one engine.

Are you asking whether you can maintain the SAME heading and track as before the engine failure, or whether you can maintain A heading and track?  Because yes, you will be able to maintain a heading and track, but it obviously won't be the same!  And of course you will have sideslip with an engine out and no rudder input.  If you understand what sideslip is, and you know what a rudder does, why would you even ask such a question?!  DV for asking a question where the knowledge indicated w/in the question is completely out of synch with what is being asked.  (i.e. unclear, not useful)

...sorry, I'm not being rude here, but really you seem to have enough basic knowledge to answer your own question.  (with an engine out you get a bunch of yaw!)  If you can clarify what specific confusion you have about when and why to use rudder vs ailerons to make this question useful I would be happy to remove my DV.

I'd say if you lost one engine and your rudder, you either hope you have enough altitude to bail out, or start praying. If the ground underneath is fairly flat, I would idle or feather the second engine and look for a place to make an emergency landing.

Found a link that gives an answer, at least for some a/c-- see page 5 here https://www.cast-safety.org/pdf/5_asymmetric_flight.pdf

What will happen is the airplane will yaw into the dead engine, with a certain amount of roll into the dead engine, depending on how strong the dihedral effect, the roll-yaw couple, is.  A swept wing airplane will want to roll upside down and it will take lots of aileron just to keep it right side up.
A straight wing airplane with moderate dihedral effect will require some moderate amount of aileron to keep wings level, and with wings level you will find yourself in a flat skidding turn into the dead engine with some aileron applied in the opposite direction - as if you were flying with both engines and applied rudder with opposite aileron to get the same effect.
To stop the turn you will need to introduce side slip that offsets the lateral lift coming from the skidding fuselage, and this side slip needs to be induced by bank, so enough extra aileron will be needed to lower the wing into the live engine until the change in heading stops.  You normally lower the wing into the live engine about 5 degrees even with rudder being used normally, but in this case it'll take a lot more.
You will now find yourself flying more or less straight, that is, not turning, but with the nose skewed toward the dead engine, and quite a lot of bank into the live engine, whatever it takes to stop the skidding turn. You will be all askew but more or less proceeding in a single direction and it will be more or less the same as if you just applied rudder with opposite aileron induced bank to stop the skidding turn caused by the rudder application.
Adverse yaw from the aileron application will aggravate the situation since the down aileron is on the dead engine side, so adverse yaw is helping the live engine.  And obviously, more power aggravates the condition compared to less power and you might find that wide open throttle causes more skidding than can be offset with bank and you run out of aileron and are force to reduce power.
So it might take most of the aileron travel to maintain this configuration, and of course the drag is so high that if you are in a piston twin you may not be able to maintain altitude at any height, even if you have ample power, because you might be forced to reduce power to keep the thing from rolling over as you run out of aileron.
If you're in something like a Seminole at 5000 ft, you might be able to maintain level flight in that condition, maybe if down around blueline speed, but I wouldn't be surprised if the best that you can do is a shallow descent as you're forced to lower the nose to keep from stalling in that skewed profile.
If you're taking multi-engine training, there's only one thing to do; go try it and see what happens and report back here.

You can use ailerons in cruise, but it gets dangerous if airspeed decreases towards stall (aileron reversal and live engine will cause roll/yaw into the dead engine).
With ailerons there will be a slip towards the live engine, which helps counteract the yaw away from the slip.  The configuration will very much resemble a forward slip, with the accompanying increase in drag.
As the plane is banked and side slipping, an increase in throttle and some nose up trim will also be required.
Depending on the aircraft model, additional rudder (more coordinated) may work better than ailerons alone.

The last paragraph shoud be deleted, as q defines no rudder rudder control. It is also somewhat misleading as it implies it is ok to fly some planes in sideslip during eo situations. It generally is not...

You want to ensure you make it to a safe landing spot with minimal fuel consuption. Remember, enough fuel means you have time, and in emergenciens, having time is luxury.

And after reading the reference by mins, I stand uncorrected. The question specifically asks about a situation " without any control on rudder". What I mean with the latter part of my previous comment: rudder input is necessary in eo situations (unless there is an automatic system providing it). I am unaware of multiengine acft that fly better without it. In engine out situations in multiengine acft you do not need to start lookin outside for spots to land, you check your map/gps for closest suitable airfield, and calmly proceed there, in an controlled manner.

@jpe61 well, I'll settle for a better understanding, which you seem to have.  Please keep in mind a pure aileron input will produce a slip.  In this case a forward slip (asymmetric thrust does the "rudder" part, ailerons hold the course).  As you correctly pointed out, the last paragraph does not address the question, but was added as a sensible alternative (coordinated) to handle the situation as per reference.


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How to determine the traffic avoidance manuver?
Scenario A: I started the takeoff roll and then noticed another aircraft is crossing the runway in front of me, how could I determine what is the best avoidance maneuver, like full braking to abort the takeoff? Or proceed take off and leave the ground before the collision? Or change the aircraft heading just like a car driver to avoid the traffic, in which my aircraft could probably rushed out of the runway?
Scenario B: I am holding on the runway and ready for takeoff and another aircraft is on the final to land, should I initiated takeoff ASAP or exited the runway? If I initiated the takeoff and the landing aircraft also take avoidance action and do a go around, will things getting worse since we may collide in air?

Which type of aircraft (general aviation single engine or big heavy jet) do you have in mind when asking your question? Also - controlled or uncontrolled field?

I am in a controlled field, will there be difference for a GA aircraft and a commercial aircraft?

In both of those cases, the last thing I would try to do is takeoff.

Scenario A: It all boils down to remaining distance available until impact and aircraft performance.

If you are in a commercial airliner you normally only have the option to abort. Your take-off performance calculation is normally optimized for the minimum power to get off the ground while staying safely within all applicable limits. Saves fuel and engine maintenance in the longterm. So you'll need most of the runway to get off the ground safely. Therefore no way to get airborne early (mass takes time to accelerate) and fly over the intruder.
If you are in an GA aircraft, things look a bit different at first glance but your chances of stopping before hitting the intruder are way better than gambling that your performance is good enough to fly over the intruder. Why? Lower mass and lower speeds. Normally you don't need the whole runway to get airborne which might lead to your thinking "I can get airborne and fly over him". There is just no reasonable way to make this judgement. So just try to stop, your stopping distance will be way shorter than what you are used to when you land as you are way slower.

In any case: your safest decision is to abort the take-off, slam on the brakes and, if your relative bearing to the intruder aircraft remains steady, it's time to head for the grass. Better a damaged gear than a collision.
Rule of thumb: If the aircraft owner can yell at you why you have damaged his aircraft it has been a good day. It means you are still alive.
Scenario B: Don't move. If the controller and the pilot of the approaching aircraft didn't notice and/or verbalize it on the radio make a radio transmission to give them a heads up. Something like "Bonanza 5545Y is holding position on runway 09." This might alert the approaching traffic as well as the controller.
To bring your example to the extreme and the approaching traffic doesn't go-around and gets uncomfortably close to you and you are under the impression you are in imminent danger get off the runway and, time permitting, make a radio transmission along the lines "Traffic approaching runway 09, runway is blocked, go-around, I say again, runway 09 is blocked, go-around." You can't out-accelerate him so vacate the runway. If you try to get airborne ahead of him there will be two aircraft with inadequate separation in a very dynamic environment. Try to keep it simple for everybody.
While I might get flamed here for non-standard phraseology even the FAA/your-governing-body-of-choice mandates that you should do everything that you deem helpful in this moment to avoid an accident.

There is also the possibility that the approaching aircraft will decide to land anyway but touch down further down (being highly irresponsible when doing so). So it is still much safer to stay stationary at the start of the runway.

If the aircraft takes no avoidance maneuver and proceed with the landing, what is the better avoidance maneuver for me, holding position on the runway or starting takeoff roll ASAP or try to exit the runway?

@VvV Aircraft should be crossing the threshold at 50 ft, so unless you're in an airliner, they should clear you easily.

@StephenS  Then what if I already started the takeoff roll, execute a takeoff abortion or speed up to leave the runway?

@VvV There are so many variables, it's hard to say.

In Scenario A there is no obvious answer. You take whatever action is necessary to avoid a collision. If this were a controlled airfield there would be questions raised to the ground controller. If not, similar questions would be directed at the pilot entering a runway without notifying anyone. Even at a completely deserted airfield without any form of station any pilot entering a runway would announce it on a radio beforehand just in case!
Scenario B can be a little more specific, and is a situation that occurs reasonably often at small airfields - less so at large controlled airports. Assuming you've not just busted onto the runway, ignoring traffic on final approach you probably have priority over the approaching aircraft. But if you're a large airliner and/or at a controlled airfield its hugely unlikely you've been cleared to enter the runway to line up if something is on final approach.
If this does happen:

an airliner would follow a missed approach procedure, which would take them well out of the way of the departing aircraft
a light aircraft would perform a similar manoeuvre but commonly known as a "go around". This involves positioning so that the pilot can keep eyes on the departing aircraft, for example flying just to the right of the runway at circuit height.

Scenario B happened in real life with an airliner taking off while another was on final approach (the second half): ATC gave a go around with an immediate turn to de-conflict the two aircraft.

Happened to me personally. Was short final at  Buttonville, one of Canada's busiest airports in the 90's, and I'm listening to the controller talk to the guy lining up for takeoff. Imagine my surprise when he gives him clearance when I'm about 1000 yd back. Took me about 5 seconds to realize there was no way he was getting out in time. "FFH in the go around". I was equally disappointed about the post-GA instructions - I knew there was a guy climbing into me but couldn't see him and the tower didn't say a thing. So I had to take it on my own to do a short circuit.

A) I would reject the takeoff as (I assume this is a light GA aircraft) the stopping distance is likely to be short, more specifically shorter than the distance required to continue takeoff. If you are at or very close to rotation speed it might be better to continue.
B) You should stay put. The runway is occupied and the other aircraft should go around. If he chooses to land, it is easier to avoid stationary plane. Worst case scenario is that you commence takeoff at the same time he goes around.
In both cases you and/or the other aircraft have already messed up so be vigilant out there.

What if I already started the takeoff roll and the other aircrafts proceed with the landing? Should proceed my taking off maneuver is better than holding position?


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For airliners, what prevents wing strikes on landing in bad weather?
For any low-wing aircraft, particularly the MD-80 or some of the private jets, CRJ's, the wings appear about 5ft or less above the ground. It seems that just few degrees of roll before touchdown would cause a wing strike on the runway and a disaster.
How do pilots and aircraft manufacturers avoid this ? It seems like with the thousands of daily flights - in all sorts of inclement weather conditions - this (fortunately) isn't a problem. (I'm not saying it's never happened, just that it seems exceptionally rare.)
So my question is, why isn't this a serious problem? I would expect turburlence and wind gusts to do all sorts of unpleasant things to aircraft, one of which might be disrupting the roll a couple degrees and planting the wingtip into the runway. What steps do pilots and aircraft designers take to mitigate this?

Pilots?  If the crosswind/gusts are too much to handle, you use another runway or go to another airport.

Remember that while for regulation the wings are bended "downwards", and the bending needs to be taken into account. However while it is landing the wings are bended "upwards" as lift is more than gravity of the wing itself, only after full touchdown with all wheels the lift becomes zero.

On heavies, as ymb1's answer shows, the wings level crab and kick method (basically air-skidding into the touchdown) is used because the mass of the airplane means you don't have the roll rate necessary to screw around safely with the wing tips close to the surface.  On the other hand, the mass gives a good delay between initiating the skid into the crosswind and the airplane starting to drift laterally (because you are in effect in a skidding turn and eventually you will start turning, which is perceived as a downwind drift) so it favours this technique.
On lighter jets like Regionals and corporate, you use light aircraft side slip technique, wing down with rudder to align with the runway.  On the CRJ IIRC, you have about 8-10 degrees of roll available before wingtip contact, which is actually quite a bit.  Side slipping to correct for a crosswind when you are at 125-135 kt rarely requires more than, say, 5 degrees of bank.
If a crosswind is strong enough to require more than 5 degrees, you usually end up touching down with a bit of crab in, or you may use an extra shot of rudder to kick it straight without lowering the wing any more, and lower the nose more aggressively to plant the plane right away before it starts to drift because you have more rudder than necessary for the bank (and live with a firm touchdown).
That being said, on the CRJ fleet there are usually several wing tip strikes a year during crosswind landings in gusty conditions, where a gust dropped the wing more than expected.  The result is usually just a bit of wing tip road rash. Winglet repair schemes are a recurring event at the OEM's support organization.

"wing tip road rash". Holy moly, that's terrifying but reassuring at the same time, knowing the aircraft can take that kind of abuse if absolutely necessary.

By the time the tip touches the pilot flying already has counteracting aileron applied, just not enough soon enough, and airplanes like the RJ have really powerful roll control, so the tip is off the ground again right away.

Those wings are strong! I remember watching a documentary which showed a 777 having its wings tested to destruction (for model certification purposes). I couldn't believe how far they bent before finally disintegrating. I can quite believe that scraping a winglet is not very serious.


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Would a flight consisting of solely first-class passengers be cancelled due to center-of-gravity issues?
Consider any modern jetliner (narrow-body and widebody separately) in a standard 2 or 3 class configuration. If somehow only first-class or business-class passengers bought seats, would that plane be impossible to fly because all the weight is focused up front? And since they are premium-class they can't be moved aft for load-balancing.
If this is (practically) impossible, then doesn't that suggest there is a minimum number of economy passengers required as "ballast" for every premium passenger?
I suspect one answer might be that the flight wouldn't be profitable anyway with few-to-none economy seats sold, so it would be cancelled. But let's suppose in that case the aircraft needed to be flown anyway because it was required to be at the destination airport, profitably or not. Doesn't that suggest there is a maximum "premium-only" passenger count that would actually be less than the number of premium seats? I.e they would have to tell some premium passengers "sorry you're bumped, or you can go to economy" despite the plane being 80% empty?

You could always put some heavy cargo in the aft section of the hold...

RE: "the flight wouldn't be profitable anyway with few-to-none economy seats sold, so it would be cancelled." this is not all that matters, the plane is already scheduled for a next flight leaving from the destination. You still have to fly the plane to the destination so it can makes it's next flight.

I've read somewhere that if first class is full, the flight (at least) breaks even. Consider that fares vary by up to 100x for the same flight, and they should be heavily weighted toward the front.

I was on an IAD to LAX flight where 1st class was filled by bumping passengers up, but all "Economy Plus" passengers were re-seated close to or behind the wing. Every economy passenger still had the entire row to themselves. I learned from the woman at check-in that this was not the emptiest that flight flew, but that it also was not a daily flight, but a flight scheduled as needed to ensure aircraft rotated through a maintenance base.

Referring to economy passengers as "ballast" also helps explain a lot about how airlines treat economy passengers...

Could/Would the airline arrange for additional cargo to be loaded if the plane was flying with plenty of excess capacity?

@Criggie Yes, that's common practice.

@Bristol Do you know what the aircraft type was?

@Terry It is an Airbus A318. I have actually been on this flight, an amazing experience.

@bta Economy passengers? Oh, you mean the self-loading freight!

It would be cancelled due to anti-snob regulations.

Even if it was impossible, I'm sure they'd sooner fill the economy class with rocks than cancel the whole flight.

I once had a 747 flight from Munich to Amsterdam where I and 2 colleagues where the ONLY passengers on the flight. We had booked economy but the crew asked us to come sit business so they didn't need to walk around so much to serve us. (We got re-scheduled to a different plane due to bad weather causing delays and re-routes. Our luggage ended up in Moscow as the plane we originally should have had(a 737) was rescheduled there and the luggage was already loaded before the re-schedule.) Good thing my car- and home-keys where in in my jacket, not in my luggage. Took days to get the suitcase back

It is much more difficult to load a plane so that the center of gravity ends up being too far forward than too far aft.
Excessive weight forward can almost always be counteracted by increased trim on the stabilizer.
When small GA planes crash due to Balance, its almost always too much rear-weight, not too much forward-weight.
Especially in the case you describe, the pax probably have luggage, and the luggage can be loaded in the rear of the plane for balance.

reminds me of the anecdote "Nose heavy planes fly poorly, tail heavy planes don't fly at all"

As an example of one effect of forward (but not out of limits) CG: My instructor and I could never get a proper spin in a 2-32.  He was a big guy, and having him in the rear seat moved the CG well forward..  With the forward CG, there wasn't enough aft stick to keep the wing stalled.  The plane would break the stall all by itself.  Flying solo, I had to add ballast to get the CG forward enough to be in limits, so I'm sure I could have done a spin solo.  But of course I never tried it solo.

Are you sure the first sentence is correct? To me it sounds contradictory to everything else in your post.

I re-read my first sentence and stand by it.  It is very hard to load a plane too far forward.  You could literally pack the cockpit with dumbbells, and still manage to fly properly.  You can easily and accidentally load a plane too far aft.

Would the location of 1000kg (ten 100kg passengers) even be noticeable on a 60,000kg plane (737)? Say if they all walked from the first 4 to the last 4 rows? I remember flying in a C130 and many of us were up walking around during a lot of the flight - until we corkscrewed into BGW.

@abelenky: If a plane's center of gravity is ahead of the center of lift and its airspeed is insufficient to control pitch, it will nose down until airspeed has increases enough to gain pitch control.  Moving the COG forward will increase the minimum airspeed needed for pitch control, but pitch control will be stable.  If COG is behind COL and airspeed gets too low, the plain will nose up, reducing airspeed further, causing the plane to nose up further, until the airspeed drops so low the plane falls out of the sky without ever regaining useful forward airspeed.

@abelenky your grammar is being misinterpreted. The first time I read your first line, I thought it meant "A plane with a forward CG is difficult to load weight into" rather than "it is difficult to load weight such that the CG is too far forward", which is what you mean.

Yes, the wording of the first sentence is misleading.  It would be good to replace it with a clear unambiguous statement.

@supercat, your comment would make up a fine answer!

@AnoE: The comment offers an explanation of why being nose heavy is qualitatively less of a problem than being tail heavy.  I wouldn't want to post an answer, however, without quantitative knowledge of how much the weight distribution would affect things like takeoff distance, climb rate, fuel economy, etc.  Weight distribution will affect drag, and the ability of a plane can fly safely with a less-than-ideal weight distribution wouldn't mean that a plane's fuel economy could never be improved by adding ballast if there was no paid cargo available.

As has been noted in the previous answer and comments to date, you can compensate (if you need to) for a first class loaded with pax but with the rest of the cabin empty by putting the pax bags in the rear.
Generalized statements like the above sometimes have exceptions, but I'd be surprised if that one did.
I worked a weight and balance for a 747-400F putting the equivalent of 72 pax at 200 lb each in the forward quarter of the airplane. That moved the c.g. %mac from 30.8 to 22.4, still well aft of the zero fuel weight forward c.g. limit of 16.0.
Then I put in 200000 lb of fuel, 2000 for taxi, 170000 burn. The forward c.g. limits for the taxi envelope, takeoff envelope, and landing envelope were still not exceeded.
The moral of the story thus far is that passengers take up a lot of room, but they don't weigh much relatively speaking.
However, when I loaded 350000 lb of fuel, it put the taxi c.g. at 10.9, just forward of its limit of 11.0, and the takeoff c.g. at 10.8, just forward of its limit of 11.0. I fixed the out of limits condition by putting in 50 lbs of baggage for each pax, splitting the weight between K48L and K48R, the two aftmost lower cargo ULD positions.
If you want to take a look at the test load with the 350000 lb of fuel but before adding in the bags, go to https://terryliittschwager.com/WB/index.php. Once there and after dismissing the initial message window, select the SERVER Load at the top of the list. It has a false date of 2222-01-08 to keep it at the top.
You can fix the out of limits condition by scrolling down to K48L and K48R and putting 1800 lb in each.

This is only anecdotal evidence, as it was 20 years ago and I don't remember or have access to any specific numbers, but when I was testing load&balance program we have developed for 747-300, when it was almost fully loaded (with fuel etc) and perfectly balanced (middle of envelope), moving 20 pax from one very end of the plane to another end was causing it to exceed allowed envelope.

This is an incredible real-world answer!  Thanks man!

@ArturBiesiadowski Do you happen to recall which of the envelopes it was in the middle of and which envelope was exceeded by moving the 20 pax? The only way I can think of that the movement of 20 pax would move one envelope out of limits when before the movement one envelope was in the middle would be that the zfw c.g. was next to a limit while the takeoff c.g. was in the middle.

The passengers don't weigh enough to seriously affect the weight & balance, especially since there are usually less than 20 people in first class.  I was on a Pan Am flight soon before its demise, and there were maybe 10 people on the plane.  They invited everyone up to first class, after all the baggage had been loaded, so there was no compensating for the shifted passenger load.

Interesting. My mom took Flight 800 two days after the infamous explosion and she told me the same story. The flight was almost empty and the crew was in shock and mourning the loss of coworkers. They invited everybody to first class as a result. That flight did not crash either.

For smaller aircraft (at least, for an A321), it would seem that they do.  In that incident, they re-balanced the airplane by moving six passengers to the back.

A fascinating anecdote!  Good one!